dt-bindings: serial: stm32: add wakeup option

Add a note for enabling wakeup capabilities of usart
Signed-off-by: Bich Hemon <bich.hemon@st.com>
Signed-off-by: Erwan Le Ray <erwan.leray@st.com>
Reviewed-by: Rob Herring <robh@kernel.org>

Link: https://lore.kernel.org/r/1560433800-12255-2-git-send-email-erwan.leray@st.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/st,stm32-usart.txt

@@ -20,6 +20,11 @@ Optional properties:
   linux,rs485-enabled-at-boot-time: see rs485.txt.
 - dmas: phandle(s) to DMA controller node(s). Refer to stm32-dma.txt
 - dma-names: "rx" and/or "tx"
+- wakeup-source: bool flag to indicate this device has wakeup capabilities
+- interrupt-names, if optional wake-up interrupt is used, should be:
+  - "event": the name for the interrupt line of the USART instance
+  - "wakeup" the name for the optional wake-up interrupt
+
 
 Examples:
 usart4: serial@40004c00 {